Brake &amp; clutch control for industrial track

ABSTRACT

The fluid control system applied to the industrial trucks, is provided with a check valve in a fluid line between a master cylinder and a plurality of wheel cylinders so as to interrupt the return flow of brake oil from the wheel cylinders to the master cylinder, and an interfering means in a bypass line to the check valve so as to admit the return flow of the brake oil in a controlled or restricted condition when the oil pressure in the hydraulic clutches reach to a sufficient pressure for engaging the clutches.

United States Patent Iwaoka et al. [451 Dec. 12, 1972 [541 BRAKE QCLUTCHCONTROL FOR 3,181,667 5/1965 Lohbauer et al ..l92/4 A INDUSTRIAL TRACK[72] inventors: Seijiro Iwaoka; Hiroo Mori, both of primary Examinerwi|1iam QD Japa Assistant Examiner-Randall Heald [73] Assignee: K b hikiKaisha Toynda Jldo h k- Attorney-Robert E. Burns and Emmanuel J. Lobatoki Seisakusho, Aichi-ken, Japan [22] Filed: June 25, 1970 57 ABSTRACT 1Appl 49,819 The fluid control system applied to the industrial trucks,is provided with a check valve in a fluid line I30] Foreign ApplicationPriority Dat between a master cylinder and a plurality of wheelcylinders so as to interrupt the return flow of brake oil i332: from thewheel cylinders to the master cylinder, and sc tio' -69 Jaan1131;111:711111111144/72203 an imflfeflns means in a bypass line w thecheck valve so as to admit the return flow of the brake oil in 52 s CL192 3 A, 1 352 924 A a controlled 01' restricted condition when the Oilpres- 151 1nt.C1 .nsn 67/02 Sure in the hydraulic Clutches reach to aSufficient 58 Field of Search "mm/13 A, 4 A, 4 c; 188/352, pressure forengaging the clutches- [56] References Cited 9 Claims, 5 Drawing FiguresUNITED STATES PATENTS 3,298,471 1/1967 Evans ..l92/3 R PKTENTED urn 12m2 SHEET 1 9F 4 P'ATENTEDuzc 12 1922 saw u or 4 BRAKE & CLUTCH CONTROLFOR INDUSTRIAL TRACK BRIEF SUMMARY OF THE INVENTION The presentinvention relates to an improved fluid power control system foroperating the industrial truck such as a fork lift truck and shovelloader.

Generally, it is well-known that the fluid actuated forward and reverseclutches are utilized for operating trucks easily in connection withengine driven torque convertor. For example, as disclosed in U. S. Pat.No. 2,786,368, in the industrial trucks such as fork lift truck andshovel loader, since it is required to release the clutches so as tostop driving of the truck when the engine is driven at high speed foroperating the fork lift or shovel loader and further it is desirable tocommence the above-mentioned operation just after stopping the truck byapplying the releasing motion of the clutches in accordance with thedepression of the brake pedal, it is conventional that a cut-off valveis hydraulically connected with a brake pedal by way of a fluid line andalso is communicated with the hydraulic clutch means so that the clutchmeans can be released by depressing the brake pedal.

However, in the above-mentioned control system, following two drawbackshave been experienced; that is, at the time of releasing the brake pedalfor commencing the driving of the truck, wheel cylinder assemblieshydraulically connected with the brake pedal are released in accordancewith the initial return motion of the brake pedal so that the brakingforce becomes nil and then the hydraulic clutches are engaged by leadingcompressed oil thereinto in accordance with the motion of the cut-offvalve when the brake pedal is further returned to its original freeposition. Consequently, it is inevitable to avoid the creation of a timelag 0.5 to 1 second between the time when the braking force becomes niland the time when the hydraulic clutches are engaged, so that the truckmoves 0.5 to 1 meter rearwardly by the weight thereof when the truck iscommenced its hill climbing.

On the other hand, in case of using the above-mentioned industrial truckon level land, the truck moves instantly when the hydraulic clutches areengaged so that strong shock is imparted to the operator and leadingsubstances. Therefore, the operation of the industrial truck in theabove-mentioned conditions is always carried out in dangerousconditions. To absorb the shock created at the time of driving, acertain damper for absorbing the shock energy was tried by arrangementin a fluid circuit communicated with the hydraulic clutches. However,the above-mentioned time lag is enlarged so that the distance ofrearward movement of the truck becomes large when the brake pedal isreleased in a condition of hill climbing.

To eliminate the above-mentioned drawbacks, U. S. Pat. No. 2,972,906teaches utilization of a clutch treadle which moves a spool valvethrough a suitable linkage to vary pressure on one hydraulic clutch anda brake treadle which is connected through a linkage to a brake mastercylinder. In the inching control device, according to theabove-mentioned U. S. Pat. No. 2,972,906, when the engine is driven at ahigh speed while stopping the truck so as to operate a cylinder for loadhandling operation, the hydraulic clutches to drive the truck in aforward direction or in a reverse direction must be controlled in theirreleased condition, therefore, the above-mentioned clutch pedal isdepressed. On the other hand, when it is required to start hillclimbing, the above-mentioned brake pedal is depressed and then anaccelator pedal is depressed while the depression of the brake pedal isreleasing so that the rearward movement of the truck can be prevented.However, it is practically difficult to operate two kinds of brakepedals, that is, the clutch treadle and the brake pedal, because, theoperator of the truck has to operate the accelator pedal, a changelever, a steering handle and a lever for controlling motion of a valvewhich actuates the load handling operation, beside the above-mentionedoperation of brake pedals.

The principal object of the present invention is to eliminate theabove-mentioned drawbacks of the conventional industrial trucks, inother words, to provide a fluid control system which permits easyoperation of the truck. Further, smooth starting with weak shock andsafely starting the truck without moving rearward when the operatorwants to start its hill climbing.

Generally, in the fluid control system of the present invention, a checkvalve is arranged in a fluid line from a master cylinder for brakingoperation to a plurality of wheel cylinders so that the return flow ofthe brake oil from the wheel cylinders to the master cylinder isinterrupted, and a control valve is arranged in a bypass line, so as toattain the following function. That is, when the pressure of oil led tohydraulic clutches elevates to a sufficient pressure for engaging theclutches, the control valve admits to communicate the bypass line sothat the brake oil can flow back from the wheel cylinders to the mastercylinder. Consequently, since the clutches are engaged and the drivingforce of the engine is transmitting to driving wheels before theclutches are engaged, the truck is commenced to move in accordance withreleasing of the braking force and it can be prevented to move the truckbackward by its own weight at the start of hill climbing. Further, sincethe control valve, which is arranged in the bypass line so as to releasethe braking force, has a function of a throttle to restrict the backflow of the brake oil from the wheel cylinders to the master cylinder,the oil pressure in the wheel cylinders decreases gradually even thoughthe depression of the brake pedal is released instantly, so that it canbe prevented from starting the vehicle, instantly.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING FIG. I is adiagrammatic view of an embodiment of fluid control line for theindustrial truck according to the present invention,

FIG. 2 is a sectional view of the device in connection with the fluidcontrol line shown in FIG. 1,

FIG. 3 is a sectional view of a modified embodiment of fluid controldevice for the industrial truck, according to the present invention,

FIG. 4 is a diagrammatic view of a further modified embodiment of fluidcontrol line for the industrial truck, according to the presentinvention,

FIG. 5 is a sectional view of a modified embodiment of fluid controldevice in connection with the fluid control line shown in FIG. 4.

3 DETAILED DESCRIPTION OF THE INVENTION In an embodiment of the fluidpower control system according to the present invention, shown in FIG.1, a fluid control hydraulic line comprises a master cylinder 2 operatedby a brake pedal 1 and a check valve 3 in a hydraulic line between themaster cylinder 2 and a pair of wheel cylinders 5 so that an inversecurrent of brake oil from the wheel cylinder 5 to the master cylinder 2is prevented and a control valve 4 permits a bypass current of the brakeoil. In another hydraulic line, compressed oil is fed from a pump 7which is driven by an engine 6 to a torque convertor 13 via a checkvalve 14 while the compressed oil is fed to a pair of oil clutches 10via a cut-off valve 8 and a change valve 9 so that turbine oil, foroperating the torque converter 13 and the hydraulic clutches 10, iscontrolled. When the pressure of the compressed oil fed from the pump 7becomes a level higher than a predetermined value, the compressed oil isdischarged through a waste valve 12. The hydraulic clutches 10 areoperated by a manual operating lever 11 connected to the change valve 9.

For example, FIG. 1 shows that the lever 11 is positioned at its neutralposition and the clutches 10 are connected to a tank so that the oilpressure is maintained at zero level, in other words, the clutches 10are maintained at released condition. However, when the lever 11 isturned to right (in FIG. I), the change valve 9 is changed to aconnection of a right side position and the left side clutch 10 isconnected to the pump 7 so that the left side clutch I is engaged. Onthe other hand, the right side clutch 10 is connected to the tank sothat this clutch I0 is released. For example, in the case that the leftside clutch 10 is connected to a reduction gear for forward movement ofthe truck while the right side clutch 10 is connected to a reductiongear for backward movement thereof, the truck is moved forward by aturning motion of the lever 11 to a right side in FIG. I. On the otherhand, the change valve 9 is changed in its connection to a left side oneby a turning motion of the lever II to a left side so that the truck ismoved backward. Changing motion of the cut-off valve 8 is controlled byhydraulic pressure created by the master cylinder 2 and the hydraulicpressure is transmitted to the cut-off valve 8 via a conduit 15.Position of the control valve 4 is controlled by hydraulic pressuretransmitted from the clutches l0 and this hydraulic pressure is led tothe control valve 4 via a conduit 16.

MOre detailed constructions of the control valve 4 and the cut-off valve8 are shown in FIG. 2. The check valve 3 is arranged in a hydraulic line17 between the master cylinder 2 and the wheel cylinder so that a returncurrent of brake oil from the wheel cylinder 5 to the master cylinder 2is prevented and a spool 19 is movably disposed in the control valve soas to control the fluid current in a bypass line 18. The spool 19 isalways pushed to the right (in FIG. 2) by a helical spring 21 so thatthe force of spring 21 is always against a force directed to a left handdirection which is created by the turbine oil under pressure led into acompression chamber 19b through a conduit 16. When the pressure of theturbine oil is elevated to more than a predetermined valve, the spool 19is displaced to the left while opposing the force of spring 21. Anannular groove 19a, formed upon the spool 19, communicates with apassageway 18 so that the interruption of the hydraulic line from thewheel cylinders 5 to the master cylinder 2 is released. An opposingforce to the spring 21 is created by a retainer 20 disposed in thecontrol valve 4. An annular spacer 27 is disposed between the retainer20 and an end of the control valve 4. An annular spacer 28 is disposedbetween the retainer 20 and the spring 21. The length of stroke (A) ofthe spool 19 can be adjusted by changing the thickness of the spacer 27,since the spool 19 contacts with the spacer 28 at the termination of thestroke thereof. An end portion of the spool 19 coaxially extends to anoutside of the retainer 20 and engages with double nuts 29 and 30 sothat relative positions between the annular groove 19a and thepassageway, that is, bypass line 18 is adjustable.

Next, the construction of the cut-off valve 8 is hereinafterillustrated. A spool 22 is movably disposed in the cut-off valve 8 so asto control the hydraulic line between the pump P and the hydraulicclutches 10 and is always pushed to the left (in FIG. 2) by the force ofspring 23 so that the rightward force created by brake oil underpressure is opposed. The above-mentioned brake oil is led into a chamber24 by way of a conduit 15. When the hydraulic pressure of the brake oilelevates to more than a predetermined valve, the spool 22 is displacedrightward while opposing the force of spring 23, so that the hydraulicline from the pump P to the clutches 10 is interrupted and the clutches10 are subjected to communicate with the tank, therefore these clutches10 are disengaged. After rightward displacement of the spool 22 by itsstroke D, the spool 22 contacts the body of cut-off valve 8.

The leftward displacement of the spool 22 is restricted by a retainer 26mounted on the body of cutoff valve 8. To prevent mixing of the brakeoil with the turbine oil, an oil seal 25 is adopted to the spool 22.

When the engine is driven and the manual lever 11 is turned to the rightso that the truck is in a condition of forward driving, the left sideclutch l0 communicates with the pump 7 under pressure, while oil underthe same pressure communicates with the chamber 19b by way of thehydraulic conduit 16, so that the spool 19 is displaced to the left.Consequently, the hydraulic passageway from the wheel cylinders 5 to themaster cylinder 2 is opened. In this condition, if the brake pedal 1 isdepressed so as to stop the driving of the truck, the brake oil of themaster cylinder 2 pushes the check valve 3 so that interruption of thehydraulic line between the master cylinder 2 and the wheel cylinders 5is released, thereby a braking force is imparted to the truck while thepressure of the brake oil is admitted to the chamber 24 of the cut-offvalve 8 through the hydraulic conduit 15 so that the spool 22 is pushedto the right (in FIG. 2), thereby communication between the pump 7 andthe clutches I0 is interrupted while the clutches l0 communicate withthe tank so that the respective clutches are disengaged. Consequently,when it is required to actuate a cylinder for load handling operation,since the engine can be driven at a high speed while depressing thebrake pedal and it is not necessary to return the manual lever 11 to theneutral condition thereof, the stopping operation of the truck and thesuccessive load handling operation can be simply and practically carriedout. In this condition,

since the hydraulic conduit 16 communicates with the tank, a forcepushing the spool 19 to the left becomes nil, so that the spool 19 isreturned to a position shown in FIG. 2. Therefore, even if thedepression of the brake pedal 1 is released so as to drive the truckforward, since the hydraulic line from the wheel cylinders 5 to themaster cylinder 2 is interrupted by the spool 19, it is prevented torelease the braking force rapidly. However, the brake oil in the chamber24 is led to the master cylinder 2 by the resilient force of the spring23 disposed in the cut-off valve 8 so that the spool 22* is displaced tothe left and the interruption of the hydraulic conduit from the pump 7to the clutches 10 is opened as shown in FIG. 2 and the hydraulicconduit from the clutches 10 to the tank is interrupted. Consequently,oil under pressure is again admitted into the chamber 19b in accordancewith the elevation of oil in the clutches l0 and when theabove-mentioned oil pressure elevates more than a predetermined pressurecreated by the spring 21, the spool 19 is displaced to the left so thatthe wheel cylinders 5 communicate with the master cylinder 2, therebythe braking force working upon the vehicle is released.

Since the braking force imparted to the truck is released after the oilpressure in the clutches l0 elevates and the driving force of the engineis transmitted to driving wheels, it is practically prevented frommoving rearward by the dead weight of the vehicle at the start of hillclimbing. As already illustrated, the cross-sectional area of the bypasshydraulic line 18 can be properly adjusted by adjusting the thickness ofthe spacers 27, 28 and the setting position of the double nuts 29 and30. In the case of carrying fragile substances such as glassware orceramics, it is desirable that the shock created by the starting of thetruck is as small as possible. To attain the above-mentioned purpose, itis preferable to use a thick spacer 28 or thin spacer 27 so that thestroke of the spool 19 is made small. In other words, the annular groove19a partially coincides with the bypass hydraulic line 18 so that flowresistance is increased. Consequently, the brake oil gradually returnsfrom the wheel cylinders 5 to the master cylinder 2 so that the truckstarts its hill climbing gradually even though the depression of thebrake pedal is released instantly.

On the other hand, when the engine is stopped while depressing the brakepedal 1 and then the above-mentioned depression is released, since thepump 7 is maintained in its stopped condition, fluid pressure to thechamber 1% is not admitted so that the spool 19 is displaced to theright by the force of spring 21. Consequently, the braking force of thewheel cylinders 5 is maintained so that the abovementioned force can beeffectively utilized as a braking force for parking the vehicle.

In the case of hauling a vehicle while stopping the engine thereof, itis practically possible to release the braking force imparted to thetruck by means of fastening the double nuts 29 and 30 so that the bypassline 18 is communicate with the annular groove 19a of the spool 19.

Further, the force of spring 21 may be adjusted so that the spool 19 iscommenced to displace to the left (in FIG. 2) when the fluid pressure inthe driving system reaches a sufficient value to engage the clutches.However, since it is practically possible to transmit the driving forceeven though the clutches are not in their complete engaged condition butin their contacting condition so as to slide, the force of the spring 21may be adjusted so that the fluid pressure in the hydraulic systemimparted into the chamber 1% is sufficiently large to create theabove-mentioned contacting condition of the clutches when theinterruption of the communication between the wheel cylinders 5 and themaster cylinder commences to be release.

In the above-mentioned embodiment, the control valve 4 and the cut-offvalve 8 are separately applied to lead a pilot fluid pressure by way ofthe hydraulic conduits l5 and 16. However, it is practically possible toutilize the control valve 4 and the cut-off valve 8 as a single sequencevalve of one body. Further, the cut-off valve 8 is controlled inaccordance with the fluid pressure in the master cylinder 2, but it maybe practically possible that as it is well applied in the conventionalindustrial trucks, the spool 22 of the cut-off valve 8 is connected tothe brake pedal 1 by a suitable link mechanism so that the cut-off valve8 is operated in accordance with the turning of the brake pedal 1.

in a modified embodiment of the fluid control system shown in FIG. 1, acheck valve is arranged in a hydraulic line from a master cylinder forbraking action to wheel cylinders so that the check valve interrupts thereturn flow of the brake oil from the wheel cylinders to the mastercylinder, and a control valve is arranged in a hydraulic line which isbypassed to the check valve, and the control valve is communicated witha cut-off valve arranged in a hydraulic line for leading turbine oil toclutches which are operated by hydraulic pressure, so that after thecut-off valve opens the hydraulic line from a pump to the clutches, thecontrol valve opens the hydraulic line which admits the return flow ofthe brake oil from the wheel cylinders to the master cylinder.Therefore, turbine oil is supplied to the clutches before the brakingforce of the truck reaches nil, so that the driving force of the engineis transmitted to the wheels so as to start the truck at the same timeof releasing the braking force. Consequently, it is practicallyprevented to move the truck backward at the start of hill climbing.Further, it is possible to throttle the control valve so that thebraking force can be gradually released, thereby the rapid starting ofthe truck can be prevented.

Referring to FIG. 3, a plunger 2b pushes a piston 2a to the left bydepressing the brake pedal 1 downward. Brake oil is discharged from themaster cylinder 2 and led to the check valve 3 via the hydraulic line 17so that the check valve 3 is pushed and the hydraulic line is opened,thereby the wheel cylinders 5 create a braking force which is impartedto the truck. And the spool 19 is moved to right by the depression ofthe brake pedal 1. Consequently, the communication of the annular groove19a of the spool 19 with the hydraulic passageway 18 is interrupted sothat the return flow of brake oil from the wheel cylinders 5 to themaster cylinder 2 is interrupted, while the hydraulic line from the pump7 to the clutches 10 is closed by the spool 19, thereby turbine oil inthe clutches 10 is discharged into the tank so that the clutches 10 aredisengaged. Therefore, in the case when the lever 11 is turned to theright and the truck is in the forward driving condition and since thebraking force is applied to the truck by depressing the brake pedal 1and then the clutches 10 are disengaged, it is possible to carry out theload handling operation while driving the engine at a high speed withoutreturning the manual lever 11 to its neutral position.

Next, when the depression of the brake pedal 1 is released, the brakepedal 1 is turned to its original position by the force of spring 31 andthe spool 19 is displaced to the left as shown in FIG. 3. In thismotion, the plunger 2b is returned to its position, shown in FIG. 3,prior to the piston 20, the piston 24 is returned to its position shownin FIG. 3 after the spool 19 is displaced to the left so that brake oilfrom the wheel cylinders 5 is led to the master cylinder 2 via thehydraulic passageway 18 and the annular groove 19a. It is importantthat, when the spool 19 is displaced to the left, firstly, the hydraulicline between the pump 7 to the clutches 10 is opened and secondly, thehydraulic line from the wheel cylinders 5 to the master cylinder 2 isopened. By the above-mentioned particular design of the system, since itis possible to release the braking force by the following manner whenthe clutches 10 are firstly engaged by feeding oil under pressurethereinto and the brake oil is secondly discharged from the wheelcylinders 5, it is practically possible to prevent the backward movementof the truck when the truck commences its hill climbing. By changing thethickness of the spacers 27 and 28, timing to open the hydraulicpassageway from the wheel cylinders 5 to the master cylinder 2 afteropening the passageway from the pump 7 to the clutches 10 can beadjusted. Further, by properly choosing the thickness of the spacers 27and 28, the effective cross-sectional area of the bypass passageway 18can be adjusted.

In the case of carrying fragile substance such as glassware or ceramics,it is desirable that the shock created by the starting of the truck isas small as possible and it is preferable to use thick spacers 27, 28.Since the bypass passageway 18 communicates with the annular groove 19aslightly evenly, though the spool 19 is displaced to its utmost leftposition, the flow resistance of this portion is increased so that thebrake oil gradually returns from the wheel cylinders 5 to the mastercylinder 2 so that the truck starts gradually even though the depressionof the brake pedal is released instantly.

In the above-mentioned embodiment, the control valve 40 and the cut-offvalve and the spool 19 for controlling the motions of these valves arearranged as one body. It may be possible to separate these valves insuch a way that a spool of the control valve 40 is actuated inaccordance with the motion of a spool of the cut-off valve.

Further modified embodiment of the present invention is shown in FIG. 4.In FIG. 4, a check valve 3, which interrupts the return flow of thebrake oil from the wheel cylinders 5 to the master cylinder 2, isarranged in the hydraulic line between the master cylinder 2, which isactuated by the brake pedal 1 and the wheel cylinders 5, and an orifice41 is used in the bypass line. The changing of the cut-off valve 8 iscontrolled by an actuator 33 which is actuated when the pressure in thecontrol hydraulic line reaches a slightly higher level than a pressureat which a brake drum (not shown) commences to contact a brake shoe (notshown). That is, when the brake pedal 1 is depressed, the oil in themaster cylinder 2 is discharged into the wheel cylinders 5 via the checkvalve 3 and the orifice 41 so that the brake shoe (not shown) issubjected to extend toward the brake drum (not shown). When the brakeshoe commences to contact the brake drum, the oil pressure in thecontrol hydraulic line is elevated rapidly, a push rod 32 is displacedto the right while opposing the force of a spring disposed in theactuator 33. Consequently, the cut-off valve 8 is changed in itsposition to that of the left so that the communication of the pump 7with the clutches 10 is interrupted and the clutches 10 communicate withthe tank. Therefore, the clutches are automatically disengaged inaccordance with the depression of the brake pedal so that the operationof the truck at the time of braking can be easily carried out.

When it is required to start the truck forward, the manual lever 11 isturned to the right while depressing the brake pedal 1, so that thechange valve 9 is changed in its position to the right side one thereof.Next, the push rod 32 of the actuator 33 is displaced to the left by aspring force in accordance with releasing the depression of the brakepedal 1. Consequently, the cut-off valve 8 is returned to its left sideposition (in FIG. 4) and oil under pressure is rapidly fed from the pump7 to the clutches 10 so that the clutches 10 are engaged and the drivingforce of the engine is transmitted to the driving wheels of the truck sothat the truck commences to move forward. If the hydraulic pressure inthe hydraulic line for braking action is very low so that the brakingforce is almost zero, the truck starts instantly.

However, the hydraulic line from the wheel cylinders 5 to the mastercylinder 2 is interrupted by a check valve 3 while restricting its flowrate by the orifice 41 arranged in the bypass line, so that the brakingforce is gradually released. In other words, the truck can be startedsmoothly. In the case of starting hill climbing, since the braking forceis released gradually after transmission of the driving force of theengine to the driving wheels, backward movement of the truck by its ownweight can be prevented.

In the embodiment shown in FIG. 5, the cut-off valve 8 and the actuator33 are arranged in one body and if the spring 23 to return the spool inthe cut-off valve 8 is settled so that the spool 22 commences to moveright when the force of the spring 23 is larger than the hydraulicpressure at the time of commencing contact of the brake drum with thebrake shoe, the similar fu nction as the embodiment shown in FIG. 4 canbe attained.

What we claim and desire to secure by Letters Patent is:

1. In an industrial truck having a single brake pedal, a master cylinderoperated by said brake pedal, a pair of wheel cylinders for impartingbraking force to said truck by fluid supply from said master cylinder, ahydraulic line connecting said master cylinder with said wheelcylinders, a hydraulic pump driven by an engine, fluid actuated clutchmeans for driving said truck and a fluid supply line connecting saidpump with said clutch means, a fluid control system comprising a checkvalve in said hydraulic line to block return flow of fluid from saidwheel cylinders to said master cylinder, a bypass fluid line shuntingsaid check valve to permit return flow of fluid from said wheelcylinders to said master cylinder without passing through said checkvalve, control valve means for controlling said return flow through saidbypass fluid line, a cut-off valve arranged in said fluid supply linebetween said pump and clutch means, means for operating said cut-offvalve including means fluidly connecting said master cylinder and saidcut-off valve, and means operably connected with said cut-ofl' valve foroperating said control valve means.

2. A fluid control system according to claim 1, wherein said means foroperating said control valve means comprises means for transmittinghydraulic pressure for actuating said control valve means to open saidbypass fluid line when hydraulic pressure in said fluid supply line tosaid clutch means exceeds a predetermined value.

3. A fluid control system according to claim 1, wherein said cut-offvalve is operably connected with said means for operating said controlvalve means so that said bypass fluid line is opened by actuating saidcontrol valve means after said cut-ofi' valve provides communicationfrom said pump to said clutches clutch means.

4. A fluid control system according to claim 1, wherein said clutchmeans comprises a pair of clutches and wherein a manually operable valveis provided in said fluid supply line to direct fluid selectively to oneor the other of said clutches.

S. A fluid control system according to claim l,-comprising aflow-controlling orifice in said bypass fluid line.

6. A fluid control system according to claim 1, wherein said cut-offvalve means comprises a housing having a port connected with said clutchmeans, a port connected with said pump and a port connected with a fluiddischarge, a valve member movable in said housing between a firstposition in which said port connected with said pump is in communicationwith said port connected with said clutch means and a second position inwhich said port connected with said clutch means is in communicationwith said port connected with said discharge, spring means normallybiasing said valve member to said first position, cooperating piston andcylinder means for moving said valve means to said second positionagainst the bias of said spring means, said means for operating saidcut-off valve including means fluidly connecting said master cylinderand said cut-off valve comprising a fluid line connecting said mastercylinder with said piston and cylinder means for moving said valvemember to said second position when said master cylinder is operated bysaid brake pedal.

7. A fluid control system according to claim 6, wherein said controlvalve means comprises a valve housing having a port connected with saidmaster cylinder, and a port connected with said wheel cylinders, acontrol valve member movable in said housing between a first position inwhich said ports are not in communication with one another and a secondposition in which said ports are in communication with one another,spring means for biasing said control valve member normally to saidfirst position, piston and cylinder means for moving said control valvemember to said secgnd position against the bias of aids rin means, saimeans opera y connecte wit sai cu off valve for operating said controlvalve means comprising a fluid line connecting said piston and cylindermeans of said control valve means with said port of said cut-off valveconnected with said clutch means to move said control valve member tosaid second position when predetermined fluid pressure is supplied bysaid cut-off valve to said clutch means.

8. A fluid control system according to claim 7, wherein said bypassfluid line includes fixed metering means for attenuating flow of fluidthrough said bypass fluid line.

9. A fluid control system according to claim 7, further comprising atorque converter connected with said pump for driving said truck undercontrol of said clutch means.

1. In an industrial truck having a single brake pedal, a master cylinderoperated by said brake pedal, a pair of wheel cylinders for impartingbraking force to said truck by fluid supply from said master cylindEr, ahydraulic line connecting said master cylinder with said wheelcylinders, a hydraulic pump driven by an engine, fluid actuated clutchmeans for driving said truck and a fluid supply line connecting saidpump with said clutch means, a fluid control system comprising a checkvalve in said hydraulic line to block return flow of fluid from saidwheel cylinders to said master cylinder, a bypass fluid line shuntingsaid check valve to permit return flow of fluid from said wheelcylinders to said master cylinder without passing through said checkvalve, control valve means for controlling said return flow through saidbypass fluid line, a cut-off valve arranged in said fluid supply linebetween said pump and clutch means, means for operating said cut-offvalve including means fluidly connecting said master cylinder and saidcut-off valve, and means operably connected with said cut-off valve foroperating said control valve means.
 2. A fluid control system accordingto claim 1, wherein said means for operating said control valve meanscomprises means for transmitting hydraulic pressure for actuating saidcontrol valve means to open said bypass fluid line when hydraulicpressure in said fluid supply line to said clutch means exceeds apredetermined value.
 3. A fluid control system according to claim 1,wherein said cut-off valve is operably connected with said means foroperating said control valve means so that said bypass fluid line isopened by actuating said control valve means after said cut-off valveprovides communication from said pump to said clutches clutch means. 4.A fluid control system according to claim 1, wherein said clutch meanscomprises a pair of clutches and wherein a manually operable valve isprovided in said fluid supply line to direct fluid selectively to one orthe other of said clutches.
 5. A fluid control system according to claim1, comprising a flow-controlling orifice in said bypass fluid line.
 6. Afluid control system according to claim 1, wherein said cut-off valvemeans comprises a housing having a port connected with said clutchmeans, a port connected with said pump and a port connected with a fluiddischarge, a valve member movable in said housing between a firstposition in which said port connected with said pump is in communicationwith said port connected with said clutch means and a second position inwhich said port connected with said clutch means is in communicationwith said port connected with said discharge, spring means normallybiasing said valve member to said first position, cooperating piston andcylinder means for moving said valve means to said second positionagainst the bias of said spring means, said means for operating saidcut-off valve including means fluidly connecting said master cylinderand said cut-off valve comprising a fluid line connecting said mastercylinder with said piston and cylinder means for moving said valvemember to said second position when said master cylinder is operated bysaid brake pedal.
 7. A fluid control system according to claim 6,wherein said control valve means comprises a valve housing having a portconnected with said master cylinder, and a port connected with saidwheel cylinders, a control valve member movable in said housing betweena first position in which said ports are not in communication with oneanother and a second position in which said ports are in communicationwith one another, spring means for biasing said control valve membernormally to said first position, piston and cylinder means for movingsaid control valve member to said second position against the bias ofsaid spring means, said means operably connected with said cut-off valvefor operating said control valve means comprising a fluid lineconnecting said piston and cylinder means of said control valve meanswith said port of said cut-off valve connected with said clutch means tomove said control valve member to said second position whenpredetermined fluid pressure is supplied by saiD cut-off valve to saidclutch means.
 8. A fluid control system according to claim 7, whereinsaid bypass fluid line includes fixed metering means for attenuatingflow of fluid through said bypass fluid line.
 9. A fluid control systemaccording to claim 7, further comprising a torque converter connectedwith said pump for driving said truck under control of said clutchmeans.